Aliphatic polyesters, such as polyglycolic acid and polylactic acid, can be decomposed by microorganisms or enzymes present in nature, such as soil or sea water, so that they are noted as biodegradable polymer materials giving little load to the environment.
Among the aliphatic polyesters, polyglycolic acid is excellent in gas-barrier properties, such as oxygen gas-barrier property, carbon dioxide gas-barrier property and water vapor-barrier property, and also excellent in heat resistance and mechanical properties. Therefore, various applications or new uses of polyglycolic acid by itself or as a composite with another resin, are under development in the fields of packaging materials, etc.
An aliphatic polyester can be synthesized by dehydro-polycondensation of an α-hydroxycarboxylic acid, such as glycolic acid or lactic acid, while it is difficult to produce a high-molecular weight aliphatic polyester through this process. In contrast thereto, in order to effectively synthesize an aliphatic polyester of a high molecular weight, there has been adopted a process of synthesizing a bimolecular cyclic ester of an α-hydroxycarboxylic acid and subjecting the cyclic ester to ring-opening polymerization. For example, by ring-opening polymerization of glycolide that is a bimolecular cyclic ester of glycolic acid, polyglycolic acid is obtained. By ring-opening polymerization of lactide that is a bimolecular cyclic ester of lactic acid, polylactic acid is obtained. An aliphatic polyester can also be obtained by ring-opening polymerization of a lactone. Production process of aliphatic polyesters by the ring-opening polymerization of these cyclic esters are known from Patent documents 1-6, shown below, etc., for example. Moreover, also the present inventors, et al., have proposed a process for producing an aliphatic polyester, comprising: ring-opening polymerization of a cyclic ester to produce the aliphatic polyester, wherein a partial polymer in a molten state is continuously introduced into a biaxial stirring device to continuously obtain a partial polymer in a solid pulverized state, the partial polymer is subjected to solid-phase polymerization, and the resultant polymer is melt-kneaded together with a thermal stabilizer to be formed into pellets (Patent document 7).
In all the production processes of aliphatic polyesters by the ring-opening polymerization of these cyclic esters, it has been considered desirable to effect polymerization in an inert gas atmosphere, such as nitrogen gas, with one of few exceptions that Patent document 4 has proposed positive use of moisture in monomer including contribution of moisture in a reactor atmosphere as an initiator and an agent for regulating molecular weight of the product polyester. However, use of water having a molecular weight regulating function is not necessarily suitable for production of an aliphatic polyester having a high molecular weight and good moisture resistance.